Polarization Mode Dispersion (PMD) occurs in an optical fiber as a result of a small residual birefringence that is introduced in the fiber core by asymmetric internal stress or strain as well as random polarization coupling due to external forces acting upon the fiber. Consequently, PMD may severely impair the transmission of a signal in an optical fiber network.
It is well-known that PMD affects differently certain polarization components of an optical signal propagating through a optical fiber transmission line, such that differential time delays occur among the components as they travel through the fiber. These differential time delays may range from about 0.1 ps/(km).sup.1/2 for low-PMD optical fibers of modern manufacture to several ps/(km).sup.1/2 for single-mode optical fibers of older manufacture. Disadvantageously, the differential time delay that may result over a "long-distance" fiber-optic link, for example, a 100 km terrestrial transmission system employing single-mode fiber, due to such differential delays may be more than 20 ps, or more than 10 ps for an transoceanic link employing modern low-PMD optical fiber.
The large time delays that occur between different polarization components of an optical signal may cause significant broadening of the optical pulses propagating through an optical link. This is especially true in modern digital lightwave systems which operate at bit rates of at least 10 Gbps per transmitted-wavelength-channel. In fact, the broadening of a pulse by a different time delay of, e.g., about 20 ps, in a high-bit rate system may cause a partial closure of the "eye diagram" of the received electrical signal by about 0.5 dB, which will significantly distort a received signal.
It is well-known, however, that the differential time delay that might occur in a particular transmission fiber is not constant over time, but may vary over time as the physical environment, e.g., temperature, pressure, etc., of the fiber changes. Thus, the statistics of time-dependent differential time delay caused by PMD in optical fiber usually follows a Maxwellian distribution, and, therefore, at any point in time, may be substantially lower to several times higher than its average (or mean) value.
(Note that in some older high-PMD optical transmission fibers a differential time delay of up to, e.g., 100 ps, is theoretically possible. A time delay of that order may cause, for example, complete fading in the electrical signal, as reported in, for example, the article entitled "Polarization Effects on BER Degradation at 10 Gb/s in IM-DD 1520 km optical Amplifier System" by Y. Namihira et al, and published in Electronic Letters, Vol. 29, No. 18, p. 1654, 1993.)
Prior methods of dealing with signal impairments due to PMD in an optical fiber include, for example; (a) electrical equalization of the signal distortion caused by PMD, as discussed in the article entitled "Experimental Equalization of Polarization Dispersion", by M. A. Santoro and J. H. Winters, and published in IEEE Photonic Technology Letters, Vol. 2, No. 8, p. 591, 1990; and (b) electrical compensation of the differential time delay in the received electrical signals, as discussed in the article entitled "Polarization Mode Dispersion Compensation by Phase Diversity Detection", by B. W. Hakki and published in Photonic Technology Letters, Vol. 9, No. 1, p. 121, 1997. Such prior methods also include (a) optical compensation of the differential time delay before converting the optical signals into electrical signals, as discussed in the article entitled "Polarization-Mode-Dispersion Equalization Experiment Using a Variable Equalizing Optical Circuit Controlled by a Pulse-Waveform-Comparison Algorithm", by T. Ozeki et al, and published in the Technical Digest Conference on Optical Fiber Communication 1994 (OSA), p. 62; and (b) other forms of compensation as discussed in, for example, the article entitled "Automatic Compensation Technique for Timewise Fluctuating Polarization Mode Dispersion in In-Line Amplifier Systems", by T. Takahashi et al., and published in Electronic Letters Vol. 30, No. 4, p. 348, 1994.
Disadvantageously, such electrical equalization schemes can only compensate for a relatively small differential time delay. They also require expensive high-speed electronics. Moreover, prior art optical compensators in general cannot automatically adapt their respective compensation schemes to handle a varying differential time delay in an optical signal traveling in a fiber that is being affected by a fluctuating-random PMD. For example, the optical compensation described in the Takahashi et al. article generates a fixed optical time delay to compensate for the distortion caused by PMD in a transmission fiber. Therefore, such a scheme is limited to dealing with a relatively small range of differential time delays. As another example, although the compensation scheme described in the T. Ozeki et al. article is capable of generating a variable, adaptive differential time delay, it requires expensive high-speed electronics to analyze the shape of received waveforms and derive an error signal that may be used to drive the compensation process toward the desired differential time delay.